1. Field of Invention
This invention relates generally to downhole perforating gun assemblies and the components associated therewith. More particularly, this invention relates to the use of a composite material to construct impermeable components of a perforating gun assembly.
2. Description of the Art
A perforating gun typically is used to form perforation tunnels in a downhole formation for purposes of enhancing the production of well fluids from the formation. The perforating gun may be part of a perforating gun assembly, an assembly that may include several perforating guns and other components. The perforating gun assembly is typically positioned downhole to the desired perforating depth via a wireline or tubing, as examples. The firing of a perforating gun normally involves detonating its shaped charges, devices that create radial perforation jets when detonated to form the perforation tunnels. To detonate the shaped charges, the perforating gun assembly typically also includes a firing head to initiate a detonation wave on a detonating cord that extends to the shaped charges. In this manner, when the detonation wave reaches a particular shaped charge, the shaped charge detonates to form a corresponding perforation jet. In addition to the shaped charges and firing head, the perforating gun assembly may include several additional components.
Each perforating gun may consist of an outer cylindrical tube called a xe2x80x9ccarrierxe2x80x9d and a loading tube located inside of the carrier. The carrier acts like a pressure vessel for the perforating gun and the included shaped charges. The loading tube has two main functions: [1] to mechanically hold the shaped charges within the carrier at a certain phasing and distance, and [2] to absorb energy coming from the fragmenting case and expanding gaseous products from the shaped charges so that the damage to the carrier is reduced.
The perforating gun assembly may also include a mechanical release mechanism to release the perforating guns from the conveyance string after the perforating guns fire. Due to this release, the perforating guns fall due to gravity into the rathole of the well. The remaining perforating gun string may then be retrieved from the well.
In the past, each of the listed perforating gun assembly components has been constructed from metallic materials. However, the use of metallic materials for perforating gun assembly components has several disadvantages.
For instance, when the shaped charges are detonated, the metal loading tube expands due to case impact and explosive gaseous expansion. The amount of loading tube expansion is limited by the inner diameter of the metal carrier. As soon as the loading tube collides with the inner diameter of the carrier, the energy from the loading tube is transmitted to the carrier. The metal carrier then swells outwardly (becoming permanently deformed) under the impact of the loading tube and may fragment into pieces. The amount of outward deformation and/or fragmentation depends on the properties of the metal from which the loading tube and the carrier are constructed.
The swelling of the gun is disadvantageous for several reasons. First, the swelling of the gun increases the overall outside diameter of the carrier/perforating gun thereby increasing the chances that the perforating gun will become stuck in the wellbore as it is extracted from the downhole environment. In addition, a number of wellbores have minimum restriction diameters wherein the guns when fired cannot swell more than this minimum restriction diameter. As a result, the guns used in such wellbores are usually at least 8-10% smaller in diameter than the minimum restriction diameter of the wellbores (thereby allowing for the post-firing swell). This 8-10% compensation in diameter effectively reduces the size of the shaped charges that fit inside the carrier thereby also reducing the performance of the gun. It would thus be beneficial to the prior art to provide a perforating gun (carrier and/or loading tube) that does not swell after being fired. The prior art would also benefit from a perforating gun that can be deployed in wellbores in which the minimum restriction diameter of the wellbore equals or substantially equals the outer diameter of the non-fired perforating gun.
Likewise, the fragmentation of the gun can also be disadvantageous. If the carrier and loading tube fragment only partially, then such fragments may also present an obstruction in the retrieval of the perforating gun. If the carrier and loading tube fragment into large pieces that remain in the wellbore, such fragments may cause problems in the subsequent operation of the well particularly if they are somehow lifted into the completion of the wellbore. Therefore, it would be beneficial to the prior art to provide a perforating gun that does not produce an obstruction for its retrieval after firing. It would also be beneficial to the prior art to provide a perforating gun that does not fragment into large pieces as a result of firing.
The use of metal in each of the listed components also serves to increase the overall weight of the perforating gun assembly. It would be beneficial to the prior art to provide a perforating gun assembly that is lower in weight than corresponding metal perforating gun assemblies.
U.S. Pat. No. 5,960,894 issued to Lilly et al. has attempted to solve at least some of these problems. The Lilly Patent discloses a perforating gun comprising an outer tube and an inner structure for holding the charges. The outer tube is made from a material having a high strength and low impact resistance, such as carbon fibers, glass fiber, or combinations thereof, that are molded in an embrittled resin, a metal matrix, or a ceramic matrix. The inner structure is made from a combustible material such as nitrocellulose, wood cellulose, cardboard, fiberboard, thermoplastic, thermoset resin, thin gauge metals, structural foam, and the like. Upon detonation of the charges, the frangible nature of the outer structure causes it to fragment into small pieces, preferably less than about 3 inches in size. The inner structure is combustibly consumed by the detonation leaving ash and residue. While this design reduces the amount of debris remaining after the perforating gun is fired, the Lilly design may not function satisfactorily if the gun assembly requires a pressure tight seal against the wellbore fluids and if the gun assembly is constructed from a composite material.
Composite materials, especially polymer matrix composites, tend to be permeable to fluids (including gases), especially to fluids under the severe pressure and temperature environment present in a wellbore. When disposed in such harsh environments, microcracks and microvoids that are inevitably formed in such composite materials lead to the formation of leak paths therethrough. Thus, the Lilly gun design constructed from composite material may become filled with wellbore fluids when disposed downhole. If this happens, the shaped charges inside the carrier will not perforate. In general, a perforating gun filled with fluid is like a bomb that will extensively damage the casing of the wellbore. To prevent such occurrence, the entire perforating gun assembly (and not only the actual perforating gun) should be impermeable to wellbore fluids. It would thus be beneficial to the prior art to provide a perforating gun assembly that does not suffer from the listed drawbacks.
Thus, there is a continuing need for an arrangement that addresses one or more of the problems that are stated above.
It is therefore an objective of the present invention to provide a perforating gun assembly that:
includes a perforating gun that does not swell after being fired;
can be deployed in wellbores in which the minimum restriction diameter of the wellbore equals or substantially equals the outer diameter of the associated non-fired perforating gun;
includes a perforating gun that does not produce an obstruction for its retrieval after firing;
includes a perforating gun that does not fragment into large pieces as a result of firing;
is lighter than prior art perforating gun assemblies; and
includes components constructed from composite materials and that are impermeable to the surrounding wellbore fluids.
Other objectives of the present invention will be obvious by reading the specification and claims appended hereto.
To achieve such objectives, our invention is a perforating gun assembly that includes at least one component that is constructed from a composite material and that is impermeable to wellbore fluids. The components may include the outer carrier and/or loading tube of a perforating gun, the connecting tubing of a gun release mechanism, and the gun connector used to attach adjacent perforating guns. The composite material is designed to be very brittle under dynamic impact. The component is made impermeable to wellbore fluids by including an impermeable liner therein. The impermeable liner can be bonded to the inner or outer surface of the component or can be embedded within the component. Since it is brittle under dynamic impact, the composite component shatters into small pieces upon detonation of the perforating gun.